I did a quick test in a tiny test mp game with one stale turn and all seems to work correct. 10 items created in turn X was in my lab in turn X+2 (with stale in turn X+1).
Hmm, thank you for the input, so the most likely explanation seems to be that somebody else forged the unique item on the same turn as I have tried and got lucky that the order was executed before mine or I canceled the order without realising it.